Image encoding and decoding method with predicted and representative motion information

ABSTRACT

A method of dividing an input image signal into pixel blocks, and performing inter-prediction on the divided pixel blocks. This method includes selecting predicted motion information from a motion information buffer storing motion information in an encoded region, and predicting motion information of an encoding target block by using the predicted motion information. The method further includes acquiring representative motion information from a plurality of items of motion information in an encoded region in accordance with first information indicating a method of selecting the predicted motion information, thereby obtaining only the representative motion information.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of PCT Application No.PCT/JP2011/055504, filed Mar. 9, 2011, the entire contents of which areincorporated herein by reference.

FIELD

Embodiments described herein relate generally to a motion informationcompression method, image encoding method, and image decoding method inthe encoding and decoding of a moving image.

BACKGROUND

Recently, an image encoding method having a greatly increased encodingefficiency has been recommended as ITU-T Rec. H.264 and ISO/IEC 14496-10(referred to as H.264 hereinafter) through the cooperation of ITU-T andISO/IEC. In H.264, a prediction process, conversion process, and entropyencoding process are performed for each rectangular block (for example,each block of 16×16 or 8×8 pixels). In the prediction process, motioncompensation of performing prediction in the temporal domain isperformed on a rectangular block to be encoded (an encoding targetblock) by referring to an already encoded frame (reference frame). Inthis motion compensation, it is necessary to encode motion informationcontaining a motion vector as spatial shift information between theencoding target block and a block to be referred to in the referenceframe, and transmit the encoded motion information to a decoding side.When performing motion compensation by using a plurality of referenceframes, it is also necessary to encode reference frame numbers inaddition to the motion information. This sometimes increases the codeamount of the motion information and reference frame numbers. There arealso motion information prediction methods by which predicted motioninformation of an encoding target block is derived by referring tomotion information of a reference frame, which is stored in a motioninformation memory (Japanese Patent No. 4020789 and Yeping Su et al,“CE9: Reduced Resolution Storage of Motion Vector Data”, JointCollaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 andISO/IEC JTC1/SC29/WG11 Document, JCTVC-D072, January 2011), and thecapacity of the motion information memory for storing motion informationsometimes increases.

As an example of a method of reducing the capacity of the motioninformation memory, representative motion information is derived in apredetermined block and only the representative motion information isstored in the motion information memory in (Yeping Su et al, “CE9:Reduced Resolution Storage of Motion Vector Data”, Joint CollaborativeTeam on Video Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IECJTC1/SC29/WG11 Document, JCTVC-D072, January 2011).

Non-Patent Literature 1

There is, however, a problem that if the predicted motion informationderivation method disclosed in J. Jung et al, “Temporal MV PredictorModification for MV-Comp, Skip, Direct and Merge Schemes”, JointCollaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 andISO/IEC JTC1/SC29/WG11 Document, JCTVC-D164, January 2011 and therepresentative motion information derivation method disclosed in YepingSu et al, “CE9: Reduced Resolution Storage of Motion Vector Data”, JointCollaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 andISO/IEC JTC1/SC29/WG11 Document, JCTVC-D072, January 2011 are different,the temporal correlation of the predicted motion information reduces,and as a consequence the code amount of the motion informationincreases.

The present embodiment has been made to solve the abovementionedproblem, and has as its object to provide an image encoding apparatusand image decoding apparatus including an information compressing devicecapable of increasing the encoding efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically showing the arrangement of animage encoding apparatus according to the first embodiment.

FIG. 2A is a view for explaining a pixel block predictive encodingsequence.

FIG. 2B is a view for explaining an example of a pixel block size.

FIG. 2C is a view for explaining another example of the pixel blocksize.

FIG. 2D is a view for explaining still another example of the pixelblock size.

FIG. 3A is a view for explaining an example of a pixel block in a codingtree unit.

FIG. 3B is a view for explaining another example of the pixel block inthe coding tree unit.

FIG. 3C is a view for explaining still another example of the pixelblock in the coding tree unit.

FIG. 3D is a view for explaining still another example of the pixelblock in the coding tree unit.

FIG. 4 is a block diagram schematically showing the arrangement of anentropy encoder shown in FIG. 1.

FIG. 5 is a view schematically showing the arrangement of a motioninformation memory shown in FIG. 1.

FIG. 6A is a view for explaining an example of an inter-predictionprocess executed by an inter-predictor shown in FIG. 1.

FIG. 6B is a view for explaining another example of the inter-predictionprocess executed by the inter-predictor shown in FIG. 1.

FIG. 7A is a view for explaining an example of a prediction unit.

FIG. 7B is a view for explaining another example of the prediction unit.

FIG. 7C is a view for explaining still another example of the predictionunit.

FIG. 7D is a view for explaining still another example of the predictionunit.

FIG. 7E is a view for explaining still another example of the predictionunit.

FIG. 7F is a view for explaining still another example of the predictionunit.

FIG. 7G is a view for explaining still another example of the predictionunit.

FIG. 8 is a view for explaining a skip mode, merge mode, and inter mode.

FIG. 9 is a block diagram schematically showing the arrangement of amotion information encoder shown in FIG. 4.

FIG. 10 is a view for explaining examples of the position of a predictedmotion information candidate with respect to an encoding targetprediction unit.

FIG. 11 is a view for explaining another example of the position of thepredicted motion information candidate with respect to the encodingtarget prediction unit.

FIG. 12 is a view showing an example of a list indicating therelationship between the block positions and indices Mvpidx of aplurality of predicted motion information candidates.

FIG. 13A is a view showing an example of a reference motion informationacquisition position indicating the center of an encoding targetprediction unit when the size of the prediction unit is 32×32.

FIG. 13B is a view showing an example of a reference motion informationacquisition position indicating the center of an encoding targetprediction unit when the size of the prediction unit is 32×16.

FIG. 13C is a view showing an example of a reference motion informationacquisition position indicating the center of an encoding targetprediction unit when the size of the prediction unit is 16×32.

FIG. 13D is a view showing an example of a reference motion informationacquisition position indicating the center of an encoding targetprediction unit when the size of the prediction unit is 16×16.

FIG. 13E is a view showing an example of a reference motion informationacquisition position indicating the center of an encoding targetprediction unit when the size of the prediction unit is 16×8.

FIG. 13F is a view showing an example of a reference motion informationacquisition position indicating the center of an encoding targetprediction unit when the size of the prediction unit is 8×16.

FIG. 14A is a view showing another example of the reference motioninformation acquisition position indicating the center of an encodingtarget prediction unit when the size of the prediction unit is 32×32.

FIG. 14B is a view showing another example of the reference motioninformation acquisition position indicating the center of an encodingtarget prediction unit when the size of the prediction unit is 32×16.

FIG. 14C is a view showing another example of the reference motioninformation acquisition position indicating the center of an encodingtarget prediction unit when the size of the prediction unit is 16×32.

FIG. 14D is a view showing another example of the reference motioninformation acquisition position indicating the center of an encodingtarget prediction unit when the size of the prediction unit is 16×16.

FIG. 14E is a view showing another example of the reference motioninformation acquisition position indicating the center of an encodingtarget prediction unit when the size of the prediction unit is 16×8.

FIG. 14F is a view showing another example of the reference motioninformation acquisition position indicating the center of an encodingtarget prediction unit when the size of the prediction unit is 8×16.

FIG. 15 is a view for explaining a spatial-domain reference motioninformation memory 501 and temporal-domain reference motion informationmemory 502.

FIG. 16 is a flowchart showing an example of the operation of a motioninformation compressor shown in FIG. 1.

FIG. 17A is a view showing an example of a reference motion informationacquisition position indicating the upper left end of an encoding targetprediction unit when the size of the prediction unit is 32×32.

FIG. 17B is a view showing an example of a reference motion informationacquisition position indicating the upper left end of an encoding targetprediction unit when the size of the prediction unit is 32×16.

FIG. 17C is a view showing an example of a reference motion informationacquisition position indicating the upper left end of an encoding targetprediction unit when the size of the prediction unit is 16×32.

FIG. 17D is a view showing an example of a reference motion informationacquisition position indicating the upper left end of an encoding targetprediction unit when the size of the prediction unit is 16×16.

FIG. 17E is a view showing an example of a reference motion informationacquisition position indicating the upper left end of an encoding targetprediction unit when the size of the prediction unit is 16×8.

FIG. 17F is a view showing an example of a reference motion informationacquisition position indicating the upper left end of an encoding targetprediction unit when the size of the prediction unit is 8×16.

FIG. 18A is a view showing examples of a representative motioninformation position.

FIG. 18B is a view showing other examples of the representative motioninformation position.

FIG. 19 is a view showing examples of the centers of prediction unitshaving different prediction sizes.

FIG. 20A is a view showing examples of a representative motioninformation position when the barycenter of a plurality of referencemotion information acquisition positions of each motion informationcompression block is set as the representative motion informationposition.

FIG. 20B is a view showing other examples of the representative motioninformation position when the barycenter of a plurality of referencemotion information acquisition positions of each motion informationcompression block is set as the representative motion informationposition.

FIG. 21A is a view showing an example of a representative motioninformation position.

FIG. 21B is a view showing another example of the representative motioninformation position.

FIG. 22 is a view showing a syntax structure according to an embodiment.

FIG. 23A is a view showing an example of a sequence parameter set syntaxaccording to an embodiment.

FIG. 23B is a view showing another example of the sequence parameter setsyntax according to the embodiment.

FIG. 24 is a view showing an example of a prediction unit syntaxaccording to an embodiment.

FIG. 25 is a block diagram schematically showing an image decodingapparatus according to the second embodiment.

FIG. 26 is a block diagram schematically showing an entropy decodershown in FIG. 25.

FIG. 27 is a block diagram schematically showing a motion informationdecoder shown in FIG. 26.

FIG. 28 is a view showing an example of a hardware configuration of animage encoding apparatus and an image decoding apparatus according toeach of the embodiments.

DETAILED DESCRIPTION

An image encoding apparatus and image decoding apparatus according toeach embodiment will be explained in detail below with reference to theaccompanying drawings. Note that in the following explanation, the term“image” can appropriately be replaced by terms such as “picture”,“pixel”, “image signal”, and “image data”. Note also that in thefollowing embodiments, portions denoted by the same reference numberperform the same operation, so a repeated explanation will be omitted.

In general, according to one embodiment, disclosed is a method ofdividing an input image signal into pixel blocks, and performinginter-prediction on the divided pixel blocks. This method includesselecting predicted motion information from a motion information bufferstoring motion information in an encoded region, and predicting motioninformation of an encoding target block by using the predicted motioninformation. The method further includes acquiring representative motioninformation from a plurality of items of motion information in anencoded region in accordance with first information indicating a methodof selecting the predicted motion information, thereby obtaining onlythe representative motion information.

First Embodiment

The first embodiment is directed to an image encoding apparatus. Animage decoding apparatus corresponding to the image encoding apparatusaccording to this embodiment will be explained in the second embodiment.This image encoding apparatus can be implemented by hardware such as alarge-scale integration (LSI) chip, digital signal processor (DSP), orfield-programmable gate array (FPGA). This image encoding apparatus canalso be implemented by causing a computer to execute an image encodingprogram.

As shown in FIG. 1, an image encoding apparatus 100 according to thisembodiment includes a subtracter 101, orthogonal transformer 102,quantizer 103, inverse quantizer 104, inverse orthogonal transformer105, adder 106, reference image memory 107, inter-predictor 108, motioninformation compressor 109, motion information memory 110, and entropyencoder 112. An encoding controller 114 and output buffer 113 arenormally installed outside the image encoding apparatus 100.

The image encoding apparatus 100 shown in FIG. 1 divides each frame,each field, or each slice forming an input image signal 151 into aplurality of pixel blocks, performs predictive encoding on these dividedpixel blocks, and outputs encoded data 163. In the followingexplanation, it is assumed, for the sake of simplicity, that thepredictive encoding of pixel blocks is performed from the upper leftcorner to the lower right corner, as shown in FIG. 2A. Referring to FIG.2A, encoded pixel blocks p are positioned above and on the left side ofan encoding target pixel block c in a frame f as an encoding target.

The pixel block is an image processing unit such as a block of M×Npixels (N and M being natural numbers), a coding unit, a macro block, asub-block, or one pixel. Note that the pixel block basically means acoding unit in the following explanation, but the pixel block can alsobe interpreted as the above-described meanings by appropriatelyreplacing the terms. The coding unit is typically, for example, a blockof 16×16 pixels, as shown in FIG. 2B, but may also be a block of 32×32pixels, as shown in FIG. 2C, or a block of 64×64 pixels, as shown inFIG. 2D. The coding unit can also be a block of 8×8 pixels (not shown)or a block of 4×4 pixels (not shown). Furthermore, the coding unit neednot be square. In the following explanation, the encoding target blockor coding unit of the input image signal 151 will also be referred to asa “prediction target block”. In addition, the encoding unit is notlimited to a pixel block such as the coding unit, and it is alsopossible to use a frame, field, or slice, or a combination thereof.

FIGS. 3A, 3B, 3C, and 3D are views showing practical examples of thecoding unit. FIG. 3A shows an example in which the size of the codingunit is 64×64 pixels (N=32). N represents the size of the coding unit asa reference. A size when the coding unit is divided is defined as N, andthat when it is not divided is defined as 2N. A coding tree unit has aquadtree structure. When the coding tree unit is divided, indices areallocated to four pixel blocks in Z-scan order. FIG. 3B shows an examplein which quadtree division is performed on the block of 64×64 pixelsshown in FIG. 3A. Numbers shown in FIG. 3B represent the ordinal numbersin Z-scan. It is also possible to further perform quadtree division onone quadtree index of the coding unit. The depth of division is definedas Depth. That is, FIG. 3A shows an example in which Depth=0. FIG. 3Cshows an example of a coding unit having a size of 32×32 pixels (N=16)when Depth=1. The largest unit of a coding tree unit like this is calleda large coding tree unit or tree block, and the input image signal isencoded in raster scan order by using this unit, as shown in FIG. 2A.

On the basis of an encoding parameter input from the encoding controller114, the image encoding apparatus 100 shown in FIG. 1 performsinter-prediction (also called inter-image prediction, inter-frameprediction, or motion compensation prediction) or intra-prediction (alsocalled intra-image prediction or intra-frame prediction) (not shown) onthe pixel block, thereby generating a predicted image signal 159. Theimage encoding apparatus 100 orthogonally transforms and quantizes aprediction error signal 152 between the pixel block (input image signal151) and predicted image signal 159, generates the encoded data 163 byperforming entropy encoding, and outputs the encoded data 163.

The image encoding apparatus 100 shown in FIG. 1 performs encoding byselectively applying a plurality of prediction modes having differentblock sizes and different methods of generating the predicted imagesignal 159. The methods of generating the predicted image signal 159 areroughly classified into two types: intra-prediction that performsprediction within an encoding target frame; and inter-prediction thatperforms prediction by using one reference frame or a plurality oftemporally different reference frames.

Each element included in the image encoding apparatus 100 shown in FIG.1 will be explained below.

The subtracter 101 obtains the prediction error signal 152 bysubtracting, from the encoding target block of the input image signal151, the predicted image signal 159 corresponding to the encoding targetblock. The subtracter 101 inputs the prediction error signal 152 to theorthogonal transformer 102.

The orthogonal transformer 102 obtains a transform coefficient 153 byperforming orthogonal transformation such as a discrete cosine transform(DCT) on the prediction error signal 152 from the subtracter 101. Theorthogonal transformer 102 outputs the transform coefficient 153 to thequantizer 103.

The quantizer 103 obtains a quantized transform coefficient 154 byquantizing the transform coefficient 153 from the orthogonal transformer102. More specifically, the quantizer 103 performs quantization inaccordance with quantization information such as a quantizationparameter and quantization matrix designated by the encoding controller114. The quantization parameter indicates the fineness of quantization.The quantization matrix is used to weigh the quantization fineness foreach component of the transform coefficient, but whether to use thequantization matrix is not essential to the embodiment. The quantizer103 outputs the quantized transform coefficient 154 to the entropyencoder 112 and inverse quantizer 104.

The entropy encoder 112 performs entropy encoding (for example, Huffmanencoding or arithmetic encoding) on various encoding parameters such asthe quantized transform coefficient 154 from the quantizer 103, motioninformation 160 from the inter-predictor 108, prediction information 165designated by the encoding controller 114, reference positioninformation 164 from the encoding controller 114, and the quantizationinformation, thereby generating the encoded data 163. Note that theencoding parameters are parameters necessary for decoding, such as theprediction information 165, information of the transform coefficient,and information of quantization. For example, the encoding controller114 includes an internal memory (not shown), and this memory stores theencoding parameters. When encoding a prediction target block, theencoding parameters of an already encoded pixel block adjacent to theprediction target block are used.

More specifically, as shown in FIG. 4, the entropy encoder 112 includesa parameter encoder 401, transform coefficient encoder 402, motioninformation encoder 403, and multiplexer 404. The parameter encoder 401generates encoded data 451A by encoding the encoding parameters such asthe prediction information 165 received from the encoding controller114. The transform coefficient encoder 402 generates encoded data 451Bby encoding the quantized transform coefficient 154 received from thequantizer 103.

The motion information encoder 403 encodes the motion information 160received from the inter-predictor 108 by referring to reference motioninformation 166 received from the motion information memory 110 andreference position information 164 received from the encoding controller114, thereby generating encoded data 451C. Details of the motioninformation encoder 403 will be described later.

The multiplexer 404 generates the encoded data 163 by multiplexing theencoded data 451A, 451B, and 451C. The generated encoded data 163contains all parameters necessary for decoding, such as the informationof the transform coefficient and the information of quantization, inaddition to the motion information 160 and prediction information 165.

The encoded data 163 generated by the entropy encoder 112 is temporarilystored in the output buffer 113 after, for example, multiplication, andoutput at a proper output timing managed by the encoding controller 114.The encoded data 163 is output to, for example, a storage system(storage medium) (not shown) or a transmission system (communicationline) (not shown).

The inverse quantizer 104 obtains a restored transform coefficient 155by inversely quantizing the quantized transform coefficient 154 from thequantizer 103. More specifically, the inverse quantizer 104 performsinverse quantization in accordance with the quantization informationused in the quantizer 103. The quantization information used in thequantizer 103 is loaded from the internal memory of the encodingcontroller 114. The inverse quantizer 104 outputs the restored transformcoefficient 155 to the inverse orthogonal transformer 105.

The inverse orthogonal transformer 105 performs, on the restoredtransform coefficient 155 from the inverse quantizer 104, inverseorthogonal transformation such as an inverse discrete cosine transformcorresponding to the orthogonal transformation performed in theorthogonal transformer 102, thereby obtaining a restored predictionerror signal 156. The inverse orthogonal transformer 105 outputs therestored prediction error signal 156 to the adder 106.

The adder 106 generates a local decoded image signal 157 by adding therestored prediction error signal 156 and corresponding predicted imagesignal 159. The decoded image signal 157 is input to the reference imagememory 107 through a deblocking filter or Wiener filter (not shown).

The reference image memory 107 stores a locally decoded filtered imagesignal 158. The inter-predictor 108 refers to the filtered image signal158 as a reference image signal 158 when generating a predicted image asneeded.

The inter-predictor 108 performs inter-prediction by using the referenceimage signal 158 saved in the reference image memory 107. Morespecifically, the inter-predictor 108 derives a motion difference(motion vector) by performing a block matching process between theprediction target block and reference image signal 158. Based on thismotion vector, the inter-predictor 108 generates an inter-predictedimage by performing motion compensation (interpolation for a motion atdecimal precision). In H.264, interpolation can be performed up to¼-pixel precision. The derived motion vector is entropically encoded asa part of the motion information 160.

The motion information memory 110 includes the motion informationcompressor 109. The motion information memory 110 reduces theinformation amount of the motion information 160 by appropriatelyperforming a compression process, and temporarily stores the compressedmotion information as the reference motion information 166. As shown inFIG. 5, the motion information memory 110 is stored for each frame (orslice), and the motion information memory 110 further includes aspatial-domain reference motion information memory 501 for storing themotion information 160 on the same frame as the reference motioninformation 166, and a temporal-domain reference motion informationmemory 502 for storing the motion information 160 of an already encodedframe as the reference motion information 166. The motion informationmemory 110 can also include a plurality of temporal-domain referencemotion information memories 502 in accordance with the number ofreference frames to be used by the encoding target frame for prediction.

Also, the spatial-domain reference motion information memory 501 andtemporal-domain reference motion information memory 502 can be obtainedby logically splitting a physically single memory. Furthermore, thespatial-domain reference motion information memory 501 can store onlyspatial-domain motion information necessary for a frame currently beingencoded, and items of spatial-domain motion information that are nolonger necessary as reference information can sequentially be compressedand stored in the temporal-domain reference motion information memory502.

The reference motion information 166 is stored for each predeterminedregion (for example, each 4×4 pixel block) in the spatial-domainreference motion information memory 501 and temporal-domain referencemotion information memory 502. The reference motion information 166further contains information indicating whether the region is encoded byinter-prediction (to be described later) or intra-prediction (to bedescribed later). Also, even when a coding unit (or prediction unit) isinter-predicted by using the motion information 160 predicted from anencoded region without encoding the value of the motion vector in themotion information 160, such as in a skip mode or direct mode defined byH.264 or in a merge mode (to be described later), motion information ofthe coding unit (or prediction unit) is stored as the reference motioninformation 166.

When the process of encoding the encoding target frame or slice iscomplete, the handling of the spatial-domain reference motioninformation memory 501 of the frame is changed to that of thetemporal-domain reference motion information memory 502 for a frame tobe encoded next. In this step, to reduce the memory capacity of thetemporal-domain reference motion information memory 502, the motioninformation 160 compressed by the motion information compressor 109 (tobe described later) is stored in the temporal-domain reference motioninformation memory 502.

The prediction information 165 complies with a prediction modecontrolled by the encoding controller 114. As described above, it ispossible to select inter-prediction, or intra-prediction (not shown) orinter-prediction in order to generate the predicted image signal 159. Itis also possible to further select a plurality of modes for each ofintra-prediction and inter-prediction. The encoding controller 114determines one of the plurality of prediction modes of intra-predictionand inter-prediction as an optimum prediction mode, and sets theprediction information 165.

For example, the encoding controller 114 determines an optimumprediction mode by using a cost function indicated by equation (1)below:K=SAD+λ×OH  (1)

In equation (1) (to be called a simplified encoding cost hereinafter),OH indicates the code amount of the prediction information 160 (forexample, motion vector information or predicted block size information),and SAD indicates a difference absolute value sum (i.e., the cumulativesum of the absolute values of the prediction error signal 152) betweenthe prediction target block and predicted image signal 159. Also, Aindicates a Lagrange undetermined multiplier to be determined based onthe value of the quantization information (quantization parameter), andK indicates the encoding cost. When using equation (1), a predictionmode that minimizes the encoding cost K is determined as an optimumprediction mode from the viewpoints of the generated code amount andprediction error. As a variation of equation (1), it is also possible toestimate the encoding cost from only the OH or SAD, or estimate theencoding cost by using a value obtained by performing a Hadamardtransform on the SAD or an approximation of the value.

Furthermore, an optimum prediction mode can be determined by using atemporary encoding unit (not shown). For example, the encodingcontroller 114 determines an optimum prediction mode by using a costfunction indicated by equation (2) below:J=D+λ×R  (2)

In equation (2), D indicates a square error sum (i.e., encodingdistortion) between a prediction target block and locally decoded image,R indicates a code amount estimated by temporary encoding for aprediction error between the prediction target block and the predictedimage signal 159 of the prediction mode, and J indicates the encodingcost. When calculating the encoding cost J (to be called a detailedencoding cost hereinafter) of equation (2), a temporary encoding processand local decoding process are necessary for each prediction mode, sothe circuit scale or calculation amount increases. On the other hand,the encoding cost J is calculated based on a more accurate encodingdistortion and more accurate code amount. This facilitates maintaining ahigh encoding efficiency by accurately determining an optimum predictionmode. Note that as a variation of equation (2), it is also possible toestimate the encoding cost from only the R or D, or estimate theencoding cost by using an approximate value of the R or D. Furthermore,a hierarchical use of these costs is also possible. The encodingcontroller 114 can also previously decrease the number of candidates ofprediction modes for performing determination using equation (1) or (2),based on information (for example, prediction modes of surrounding pixelblocks or the result of image analysis) preobtained for a predictiontarget block.

As a modification of this embodiment, it is possible to further reducethe number of prediction mode candidates while maintaining the encodingperformance, by performing two-step mode determination combiningequations (1) and (2). The simplified encoding cost indicated byequation (1) requires no local decoding process unlike equation (2), andhence can be calculated at high speed. The image encoding apparatus ofthis embodiment has more prediction modes than those of H.264, so modedetermination using the detailed encoding cost is unrealistic. As afirst step, therefore, mode determination using the simplified encodingcost is performed on prediction modes usable for the corresponding pixelblock, thereby deriving prediction mode candidates.

Since the correlation between the simplified encoding cost and detailedencoding cost increases as the value of the quantization parameterdefining the roughness of quantization increases, the number ofprediction mode candidates is changed by using this property.

Next, the prediction process of the image encoding apparatus 100 will beexplained.

Although not shown, a plurality of prediction modes are prepared for theimage encoding apparatus 100 shown in FIG. 1, and the method ofgenerating the predicted image signal 159 and the motion compensationblock size change from one prediction mode to another. The methods bywhich the predictor 108 generates the predicted image signal 159 areroughly classified into intra-prediction (intra-frame prediction) thatgenerates a predicted image by using the reference image signal 158 ofthe encoding target frame (or field), and inter-prediction (inter-frameprediction) that generates a predicted image by using the referenceimage signal 158 of one or more encoded reference frames (or referencefields). The predictor 108 generates the predicted image signal 159 ofthe encoding target block by selectively switching intra-prediction andinter-prediction.

FIG. 6A shows an example of inter-prediction. Inter-prediction istypically executed for each prediction unit, and each prediction unitcan have unique motion information 160. In inter-prediction, as shown inFIG. 6A, the predicted image signal 159 is generated by using thereference image signal 158 of a block 602 in a position spatiallyshifted, in accordance with the motion vector contained in the motioninformation 160, from a block 601 which is a pixel block in an alreadyencoded reference frame (for example, an immediately preceding encodedframe) and exists in the same position as that of an encoding targetprediction unit. That is, the predicted image signal 159 is generated byusing the reference image signal 158 of the block 602 in the referenceframe, which is specified by the position (coordinates) of the encodingtarget block and the motion vector contained in the motion information160.

In inter-prediction, motion compensation at small-number-of-pixelprecision (for example, ½-pixel precision or ¼-pixel precision) ispossible, and the value of an interpolating pixel is generated byfiltering the reference image signal 158. For example, an interpolationprocess of up to ¼-pixel precision can be performed on a luminancesignal in H.264. This interpolation process can be executed by usingarbitrary filtering instead of filtering defined by H.264.

Note that in inter-prediction, it is possible to use not only animmediately preceding reference frame, as shown in FIG. 6A, but also anyalready encoded reference frame, as shown in FIG. 6B. When the referenceimage signals 158 of a plurality of reference frames having differenttime positions are stored, information indicating the time position ofthe reference image signal 158 from which the predicted image signal 159is generated is represented by a reference frame number. The motioninformation 160 contains this reference frame number. The referenceframe number can be changed for each region (for example, each picture,slice, or block). That is, different reference frames can be used fordifferent prediction units. As an example, when an immediately precedingencoded reference frame is used in prediction, the reference framenumber of this region is set to zero. When an encoded reference frametwo frames before an encoding target frame is used in prediction, thereference frame number of this region is set to 1. As another example,when the reference image signal 158 of only one frame is stored in thereference image memory 107 (when only one reference frame is stored),the reference frame number is always set to zero.

In addition, it is possible to selectively use a size suitable for anencoding target block from a plurality of prepared prediction unitsizes. For example, motion compensation can be performed for eachprediction unit obtained by dividing coding tree units, as shown inFIGS. 7A, 7B, 7C, 7D, 7E, 7F, and 7G. It is also possible to performmotion compensation for each prediction unit divided into a shape otherthan a square, as shown in FIG. 7F or 7G.

As described previously, the motion information 160 of an encoded pixelblock (for example, a block of 4×4 pixels) in an encoding target frameto be used in inter-prediction is stored as the reference motioninformation 166. Therefore, the shape and motion vector of an optimummotion compensation block and the reference frame number can be used inaccordance with the local property of the input image signal 151. It isalso possible to freely combine coding units and prediction units. Whenthe coding unit is a block of 64×64 pixels, each of four coding treeunits (blocks of 32×32 pixels) obtained by dividing the block of 64×64pixels can further be divided into four blocks. This makes it possibleto hierarchically use blocks of 16×16 pixels from the block of 64×64pixels. It is similarly possible to hierarchically use blocks of 8×8pixels from the block of 64×64 pixels. When the prediction unit isobtained by dividing the coding tree unit into four blocks, it ispossible to execute a hierarchical motion compensation process from ablock of 64×64 pixels to blocks of 4×4 pixels.

Furthermore, in inter-prediction, bidirectional prediction using twokinds of motion compensation can be executed on an encoding target pixelblock. In H.264, a new predicted image signal (not shown) is obtained byperforming two kinds of motion compensation on an encoding target pixelblock, and calculating the weighted average of two kinds of predictedimage signals. In this bidirectional prediction, the two kinds of motioncompensation will be referred to as list 0 prediction and list 1prediction.

<Explanation of Skip Mode, Merge Mode, and Inter Mode>

The image encoding apparatus 100 according to this embodiment uses aplurality of prediction modes having different encoding processes, asshown in FIG. 8. Referring to FIG. 8, the skip mode is a mode in whichonly the syntax of a predicted motion information position 954(described later) is encoded, and other syntaxes are not encoded. Themerge mode is a mode in which only the syntax of the predicted motioninformation position 954 and the transform coefficient information 153are encoded, and other syntaxes are not encoded. The inter mode is amode in which the syntax of the predicted motion information position954, differential motion information 953 (to be described later), andthe transform coefficient information 153 are encoded. These modes areswitched by the prediction information 165 controlled by the encodingcontroller 114.

<Motion Information Encoder 403>

The motion information encoder 403 will be explained below withreference to FIG. 9.

The motion information encoder 403 includes a reference motion vectoracquisition unit 901, predicted motion vector selection switch (to bealso referred to as a predicted motion information selection switch)902, subtracter 903, differential motion information encoder 904,predicted motion information position encoder 905, and multiplexer 906.

The reference motion vector acquisition unit 901 receives the referencemotion information 166 and reference position information 164, andgenerates at least one predicted motion information candidate (alsoreferred to as a predicted motion vector candidate) 951 (951A, 951B, . .. ). FIGS. 10 and 11 illustrate examples of the position of thepredicted motion information candidate 951 with respect to a targetprediction unit. FIG. 10 shows the positions of prediction unitsspatially adjacent to the target prediction unit. AX (X=0 to nA−1)indicates an adjacent prediction unit on the left side of the targetprediction unit, BY (Y=0 to nB−1) indicates an adjacent prediction unitabove the target prediction unit, and C, D, and E respectively indicateadjacent prediction units at the upper right corner, upper left corner,and lower left corner of the target prediction unit. FIG. 11 shows theposition of a prediction unit in an already encoded reference frame withrespect to the encoding target prediction unit. In FIG. 11, Colindicates a prediction unit, which exists in the same position as thatof the encoding target prediction unit, in the reference frame. FIG. 12shows an example of a list indicating the relationship between the blockpositions and indices Mvpidx of a plurality of predicted motioninformation candidates 951. Indices 0 to 2 of Mvpidx indicate thepredicted motion vector candidates 951 positioned in the spatial domain,and index 3 of Mvpidx indicates the predicted motion vector candidate951 positioned in the temporal domain. Prediction unit position A is theposition of a prediction unit of inter-prediction, i.e., a predictionunit having the reference motion information 166, in AX, as shown inFIG. 10, and the value of X is smallest at prediction unit position A.Prediction unit position B is the position of a prediction unit ofinter-prediction, i.e., a prediction unit having the reference motioninformation 166, in BY, as shown in FIG. 10, and the value of Y issmallest in prediction unit position B. When prediction unit position Cis not inter-prediction, the reference motion information 166 inprediction unit position D is replaced with the reference motioninformation 166 in prediction unit position C. When prediction unitpositions C and D are not inter-prediction, the reference motioninformation 166 in prediction unit position E is replaced with referencemotion information 166 in prediction unit position C.

If the size of the encoding target prediction unit is larger than thatof the smallest prediction unit, prediction unit position Col may storea plurality of items of reference motion information 166 in thetemporal-domain reference motion information memory 502. In this case,the reference motion information 166 in the prediction unit at positionCol is acquired in accordance with the reference position information164. The acquisition position of the reference motion information 166 inthe prediction unit at position Col will be referred to as a referencemotion information acquisition position hereinafter. FIGS. 13A, 13B,13C, 13D, 13E, and 13F illustrate examples of the reference motioninformation acquisition position for each encoding target predictionunit size (32×32 to 16×16) when the reference position information 164indicates the center of the prediction unit in position Col. In FIGS.13A, 13B, 13C, 13D, 13E, and 13F, each block indicates a 4×4 predictionunit, and a circle indicates the position of a 4×4 prediction unit to beacquired as the predicted motion information candidate 951. FIGS. 14A,14B, 14C, 14D, 14E, and 14F illustrate other examples of the referencemotion information acquisition position. Referring to FIGS. 14A, 14B,14C, 14D, 14E, and 14F, no 4×4 prediction unit exists in the position ofa circle, so the predicted motion information candidate 951 is generatedby a predetermined method such as the average or median of the referencemotion information 166 in four 4×4 prediction units adjacent to thecircle. As still another example of the reference motion informationacquisition position, the reference motion information 166 of a 4×4prediction unit positioned at the upper left corner of the predictionunit in position Col may also be used as the predicted motioninformation candidate 951. In a case other than the abovementionedexamples, it is possible to generate the predicted motion informationcandidate 951 by using any position and any method, provided that themethod is a predetermined method.

Note that if no reference motion information 166 exists, the motioninformation 160 having a zero vector is output as the predicted motioninformation candidate 951.

By the processing described above, at least one predicted motioninformation candidate 951 is output from the reference motion block. Ifthe reference frame number of the aforementioned predicted motioninformation candidate 951 is different from that of the encoding targetprediction unit, the predicted motion information candidate 951 can bescaled by using the reference frame number of the predicted motioninformation candidate 951 and that of the encoding target predictionunit.

The predicted motion information selection switch 902 selects one of theplurality of predicted motion information candidates 951 in accordancewith a command from the encoding controller 114, and outputs thepredicted motion information 952. The predicted motion informationselection switch 902 may also output the predicted motion informationposition information 954 (to be described later). The abovementionedselection can be performed by using an evaluation function such asequation (1) or (2). The subtracter 903 subtracts the predicted motionvector information 952 from the motion information 160, and outputs thedifferential motion information 953 to the differential motioninformation encoder 904. The differential motion information encoder 904encodes the differential motion information 953, and outputs encodeddata 960A. Note that in the skip mode and merge mode, the differentialmotion information encoder 904 need not encode the differential motioninformation 953.

The predicted motion information position encoder 905 encodes thepredicted motion information position information 954 (Mvpidx)indicating which predicted motion information candidate 951 is selectedfrom the list shown in FIG. 12, and outputs encoded data 960B. Thepredicted motion information position information 954 is encoded byusing fixed-length encoding or variable-length encoding generated fromthe total number of predicted motion information candidates 951.Variable-length encoding may also be performed using the correlationwith adjacent blocks. Furthermore, if a plurality of predicted motioninformation candidates 951 have identical information, it is alsopossible to encode the predicted motion information position information954 by forming a code table from the total number of predicted motioninformation candidates 951 from which the identical predicted motioninformation candidates 951 are deleted. Also, if the total number ofpredicted motion information candidates 951 is 1, it is determined thatthe corresponding predicted motion information candidate 951 is thepredicted motion information 952, so the predicted motion informationposition information 954 need not be encoded.

In addition, in the skip mode, merge mode, and inter mode, the methodsof deriving the predicted motion information candidate 951 need not bethe same, and it is possible to independently set the methods ofderiving the predicted motion information candidate 951. This embodimentwill be explained by assuming that the same method of deriving thepredicted motion information candidate 951 is used in the skip mode andmerge mode, and a different method of deriving the predicted motioninformation candidate 951 is used in the merge mode.

<Details of Motion Information Compressor 109>

First, the motion information compression process will be explained withreference to FIG. 15. In FIG. 15, the reference motion information 166of the spatial-domain reference motion information memory 501 iscompressed and stored in the temporal-domain reference motioninformation memory 502. The reference motion information 166 which isstored in the representative motion information position for each motioninformation compression block (in FIG. 15, a block of 16×16 pixels) inthe spatial-domain reference motion information memory 501 is stored inthe temporal-domain reference motion information memory 502. Whenperforming the above-described motion information encoding process, thereference motion information 166 stored in the above-described referencemotion information acquisition position is set as the predicted motioninformation candidate 951. In this case, it is also possible to assumethat the motion information compression block virtually has the samereference motion information 166, and set the reference motioninformation 166 stored in the above-described reference motioninformation position as the predicted motion information candidate 951(the same predicted motion information candidate 951 is derived).

Next, the motion information compressor 109 will be explained withreference to the flowchart shown in FIG. 16. When the process ofencoding a frame (or an arbitrary unit such as a slice or coding unit)is complete, the motion information compression 109 compresses themotion information 160 and stores the motion information 160 in thetemporal-domain reference motion information memory 502.

First, the reference position information 164 is acquired from theencoding controller 114 (step S1601), and a frame is divided into motioninformation compression blocks as compression units of the motioninformation 160 (step S1602). The motion information compression blockis a pixel block larger than a unit (typically, a block of 4×4 pixels)by which the motion information 160 is stored by the motion compensationprocess, and is typically a block of 16×16 pixels. The motioninformation compression block may also be a block of 64×64 pixels, ablock of 32×32 pixels, a block of 8×8 pixels, a rectangular pixel block,or a pixel region having an arbitrary shape.

Then, a representative motion information position is generated inaccordance with the reference position information 164 (step S1603). Asan example of generating a representative motion information position,when the motion information compression block is a block of 16×16pixels, the reference motion information acquisition position when theprediction unit size is 16×16, as shown in FIGS. 13D, 14D, and 17D, isset as the representative motion information position. Subsequently, thereference motion information 166 of the generated representative motioninformation position is set as representative motion information (stepS1604), and the representative motion information is stored in thetemporal-domain reference motion information memory (step S1605). StepsS1604 and S1605 described above are executed for all motion informationcompression blocks.

When the unit for storing the motion information 160 is an M×M block andthe size of the motion information compression block is N×N (N is amultiple of M), the capacity of the reference motion information memorycan be reduced to (M×M)/(N×N) by executing the aforementioned motioninformation compression process.

<Other Embodiments of Representative Motion Information Position>

As another example of generating a representative motion informationposition, the central position of a plurality of reference motioninformation acquisition positions may also be used as the representativemotion information position. FIGS. 18A and 18B show the representativemotion information position of each motion compression block having asize of 16×16. FIG. 18A shows representative motion informationpositions when reference motion information acquisition positions arethe positions shown in FIG. 13D. Likewise, FIG. 18B shows representativemotion information positions when reference motion informationacquisition positions are the positions shown in FIG. 17D. Symbols ● inFIGS. 18A and 18B indicate reference motion information acquisitionpositions when a prediction unit is a 16×16 block. A representativemotion information position indicated by × is placed in the centralposition (also called the barycentric position) of four reference motioninformation acquisition positions.

As still another example of generating a representative motioninformation position, it is also possible to use a reference motioninformation acquisition position for each size of a plurality ofprediction units as the reference position information 164, and generatea representative motion information position from a plurality ofreference motion information acquisition positions.

As an example of generating a representative motion informationposition, it is also possible to use a reference motion informationacquisition position for each size of a plurality of prediction units asthe reference position information 164, and generate a representativemotion information position from a plurality of reference motioninformation acquisition positions. FIG. 19 shows the center (referencemotion information acquisition position) of each prediction unit havinga prediction unit size of 16×16 or more, when a tree block is a block of64×64 pixels.

As another example of generating a representative motion informationposition, a representative motion information position can also be setby using a reference motion information acquisition position set foreach motion information compression block. FIG. 20A shows an examplewhen the barycenter of a plurality of reference motion informationacquisition positions in each motion information compression block isset as a representative motion information position. If the barycentricposition does not match the position of a 4×4 block, it is possible toset the nearest 4×4 block as a representative motion informationposition, or generate the reference motion vector 166 of the barycentricposition by using interpolation such as bi-linear interpolation.

FIG. 20B shows an example in which one of a plurality of referencemotion information acquisition positions is selected for each motioninformation compression block, and set as a representative motioninformation position.

Furthermore, FIGS. 21A and 21B illustrate examples when motioninformation compression blocks have the same reference motioninformation acquisition position in a tree block. Since all motioninformation compression blocks have the same representative motioninformation position, representative motion information positions neednot be switched in accordance with a position in the tree block. Therepresentative motion information position need not be those shown inFIGS. 21A and 21B, and can be any position such as the upper left corneror upper right corner in the motion information compression block.

As an example of generating a representative motion informationposition, a representative motion information position may also beindicated by using BlkIdx indicating positions of 4×4 blocks in a motioninformation compression block in Z-scan order. When the size of themotion information compression block is 16×16, the representative motioninformation position shown in FIG. 21A is equivalent to the position ofBlkIdx=12. Also, the representative motion information position shown inFIG. 21B is equivalent to the position of BlkIdx=15.

As another example of the motion information compression process, themotion information compression process may also include reference framenumbers in order to reduce the memory capacity for reference framenumbers. In this case, a reference frame number stored in arepresentative motion information position is stored in the memorycapacity for reference frame numbers. Accordingly, the spatial-domainreference motion information memory 501 and temporal-domain referencemotion information memory 502 shown in FIG. 5 store reference framenumbers in addition to the motion vector information.

As still another example of the motion information compression process,when the motion information compression process does not include anyreference frame number, motion vector information in motion informationin a representative motion information position can be scaled by using areference frame number, and stored in the motion information memory 110.A typical example of the scaling process is a linear scaling processbased on reference frame number zero. That is, if the reference framenumber is not zero, the linear scaling process is performed such thatthe motion vector information refers to a reference frame correspondingto reference frame number zero. The basis of the above-described scalingprocess may also be a value other than reference frame number zero. Ifdivision occurs when performing the above-described linear scalingprocess, this division can be implemented by looking up a divisionprocess table formed beforehand.

When the size of the motion information compression block is not a 16×16block, a representative motion information position is generated byusing the same processing as described above. As an example, when thesize of the motion information compression block is 64×64, a referencemotion information acquisition position in a 64×64 prediction unit isset as a representative motion information position. As another example,a representative motion information position in the 16×16 motioninformation compression block shown in, for example, FIGS. 21A and 21Bcan be scaled in the horizontal and vertical directions in accordancewith the motion information compression block size, and the scaledposition can be set as a representative motion information position.

If no reference motion information exists because a representativemotion information position is outside a picture or slice, a positionwhere reference motion information can be acquired in a motioninformation compression block, for example, the upper left corner of themotion information compression block can be set as a new representativemotion information position. Also, if no reference motion informationexists because a representative motion information position is a regionto which intra-prediction is applied, a new representative motioninformation position can be set by executing the same processing.

<Syntax Configuration>

The syntax used by the image encoding apparatus 100 shown in FIG. 1 willbe explained below.

The syntax indicates the structure of encoded data (for example, theencoded data 163 shown in FIG. 1) obtained when the image encodingapparatus encodes moving image data. When decoding this encoded data, animage decoding apparatus interprets the syntax by referring to the samesyntax structure. FIG. 22 shows an example of a syntax 2200 used by theimage encoding apparatus shown in FIG. 1.

The syntax 2200 includes three parts, i.e., a high level syntax 2201,slice level syntax 2202, and coding tree level syntax 2203. The highlevel syntax 2201 contains syntax information of layers higher than aslice. The slice is a rectangular region or continuous region includedin a frame or field. The slice level syntax 2202 contains informationnecessary to decode each slice. The coding tree level syntax 2203contains information necessary to decode each coding tree (i.e., eachcoding tree unit). These parts each further include detailed syntaxes.

The high level syntax 2201 includes sequence and picture level syntaxessuch as a sequence parameter set syntax 2204 and picture parameter setsyntax 2205. The slice level syntax 2202 includes a slice header syntax2206 and slice data syntax 2207. The coding tree level syntax 2203includes a coding tree unit syntax 2208, transform unit syntax 2209, andprediction unit syntax 2210.

The coding tree unit syntax 2208 can have a quadtree structure. Morespecifically, the coding tree unit syntax 2208 can recursively be calledas a syntax element of the coding tree unit syntax 2208. That is, onecoding tree unit can be subdivided by a quadtree. Also, the coding treeunit syntax 2208 includes the transform unit syntax 2209 and predictionunit syntax 2210. The transform unit syntax 2209 and prediction unitsyntax 2210 are called in each coding tree unit syntax 2208 at theendmost portion of the quadtree. Information of prediction is describedin the prediction unit syntax 2210. Information of inverse orthogonaltransformation and quantization is described in the transform unitsyntax 2209.

FIG. 23 shows examples of the sequence parameter set syntax 2204according to this embodiment. The flag motion_vector_buffer_comp_flagshown in FIGS. 23A and 23B is a syntax indicating whether motioninformation compression according to this embodiment is valid/invalidfor the sequence. If motion_vector_buffer_comp_flag is 0, motioninformation compression according to this embodiment is invalid for thesequence. Accordingly, the processing of the motion informationcompressor shown in FIG. 1 is skipped. As an example, ifmotion_vector_buffer_comp_flag is 1, motion information compressionaccording to this embodiment is valid for the sequence. The valuemotion_vector_buffer_comp_ratio_log2 shown in FIGS. 23A and 23B isinformation indicating the unit of the motion information compressionprocess, and is shown when motion_vector_buffer_comp_flag is 1. Forexample, motion_vector_buffer_comp_ratio_log2 indicates information ofthe size of the motion information compression block according to thisembodiment, and a value obtained by multiplying the smallest unit ofmotion compensation by ₂(motion_vector_buffer_comp_ratio_log2) is thesize of the motion information compression block. An example in whichthe smallest unit of motion compensation is a block of 4×4 pixels, i.e.,the reference motion information memory is stored for each block of 4×4pixels, will be described below. Whenmotion_vector_buffer_comp_ratio_log2 is 1, the size of the motioninformation compression block according to this embodiment is a block of8×8 pixels. Similarly, when motion_vector_buffer_comp_ratio_log2 is 2,the size of the motion information compression block according to thisembodiment is a block of 16×16 pixels. The valuemotion_vector_buffer_comp_position shown in FIG. 23B is informationindicating a representative motion information position in the motioninformation compression block, and is shown whenmotion_vector_buffer_comp_flag is 1. For example,motion_vector_buffer_comp_position may also indicate a reference motioninformation position in the motion information compression block, asshown in FIGS. 21A and 21B, or indicate a reference motion informationposition in each motion information compression block, as shown in FIGS.20A and 20B. Also, motion_vector_buffer_comp_position can furtherindicate the center of a plurality of blocks.

As another example, the validity/invalidity of prediction according tothis embodiment may also be defined for each local region inside theslice in syntaxes of layers (for example, the picture parameter syntax,slice level syntax, coding tree unit, and transform unit) belowmotion_vector_buffer_comp_flag, motion_vector_buffer_comp_ratio_log2,and motion_vector_buffer_comp_position.

FIG. 24 shows an example of the prediction unit syntax. The flagskip_flag shown in FIG. 24 indicates whether the prediction mode of acoding unit to which the prediction unit syntax belongs is the skipmode. If skip_flag is 1, it indicates that syntaxes (the coding unitsyntax, prediction unit syntax, and transform unit syntax) other thanthe prediction motion information position information 954 are notencoded. NumMVPCand(L0) and NumMVPCand(L1) respectively indicate thenumbers of predicted motion information candidates 951 in list 0prediction and list 1 prediction. If the predicted motion informationcandidate 951 exists (NumMVPCand(LX)>0, X=0 or 1), mvp_idx_IX indicatingthe predicted motion information position information 954 is encoded.

If skip_flag is 0, it indicates that the prediction mode of a codingunit to which the prediction unit syntax belongs is not the skip mode.NumMergeCandidates indicates the number of predicted motion informationcandidates 951 derived in FIG. 12 or the like. If the predicted motioninformation candidate 951 exists (NumMergeCandidates>0), merge_flag as aflag indicating whether the prediction unit is the merge mode isencoded. When the value of merge_flag is 1, the flag indicates that theprediction unit is the merge mode. If the value is 0, the flag indicatesthat the prediction unit uses the inter mode. If merge_flag is 1 and twoor more predicted motion information candidates 951 exist(NumMergeCandidates>1), merge_idx as the predicted motion information952 indicating a block of the predicted motion information candidates951 from which merge is started is encoded.

If merge_flag is 1, prediction unit syntaxes other than merge_flag andmerge_idx need not be encoded.

If merge_flag is 0, it indicates that the prediction unit is the intermode. In the inter mode, mvd_IX (X=0 or 1) indicating differentialmotion vector information contained in the differential motioninformation 953 and reference frame number ref_idx_IX are encoded, and,in the case of a B slice, inter_pred_idc indicating whether theprediction unit is unidirectional prediction (list 0 or list 1) orbidirectional prediction is encoded. In addition, NumMVPCand(L0) andNumMVPCand(L1) are acquired as in the slip mode. If the predicted motioninformation candidate 951 exists (NumMVPCand(LX)>0, X=0 or 1),mvp_idx_IX indicating the predicted motion information positioninformation 954 is encoded.

The foregoing is the syntax configuration according to this embodiment.

Second Embodiment

The second embodiment is directed to an image decoding apparatus. Theimage encoding apparatus corresponding to the image decoding apparatusaccording to this embodiment is explained in the first embodiment. Thatis, the image decoding apparatus according to this embodiment decodes,for example, encoded data generated by the image encoding apparatusaccording to the first embodiment.

As shown in FIG. 25, the image decoding apparatus according to thisembodiment includes an entropy decoder 2501, inverse quantizer 2502,inverse orthogonal transformer 2503, adder 2504, reference image memory2505, inter-predictor 2506, reference motion information memory 2507,reference motion information compressor 2508, and decoding controller2510.

The image decoding apparatus shown in FIG. 25 decodes encoded data 2550,stores a decoded image signal 2554 in an output buffer 2511, and outputsthe decoded image signal 2554 as an output image. The encoded data 2550is output from, for example, the image encoding apparatus shown in FIG.1, and input to an image decoding apparatus 2500 through a storagesystem (not shown) or transmission system (not shown).

The entropy decoder 2501 performs decryption based on syntaxes in orderto decode the encoded data 2550. The entropy decoder 2501 sequentiallyentropically decodes a code sequence of each syntax, and reproducesencoding parameters, such as motion information 2559 and a quantizedtransform coefficient 2551, of an encoding target block. The encodingparameters are parameters necessary to decode, for example, predictioninformation, information of a transform coefficient, and information ofquantization.

More specifically, as shown in FIG. 26, the entropy decoder 2501includes a separator 2601, parameter decoder 2602, transform coefficientdecoder 2603, and motion information decoder 2604. The separator 2601separates the encoded data 2550, and outputs encoded data 2651A of aparameter to the parameter decoder 2602, encoded data 2651B of atransform coefficient to the transform coefficient decoder 2603, andencoded data 2651C of motion information to the motion informationdecoder 2604. The parameter decoder 2602 decodes an encoding parameter2651A such as prediction information and outputs the encoding parameter2570 to the decoding controller 2510. The transform coefficient decoder2603 receives the encoded data 2651B, decodes the transform coefficientinformation 2551, and outputs the decoded information to the inversequantizer 2502.

The motion information decoder 2604 receives the encoded data 2651C fromthe separator 2601, reference position information 2560 from thedecoding controller 2510, and reference motion information 2558 from thereference motion information memory 2507, and outputs the motioninformation 2559. The output motion information 2559 is input to theinter-predictor 2506.

As shown in FIG. 27, the motion information decoder 2604 includes aseparator 2701, a differential motion information decoder 2702,predicted motion information position decoder 2703, reference motioninformation acquisition unit 2704, predicted motion informationselection switch 2705, and adder 2706.

The encoded data 2651C of motion information is input to the separator2701, and separated into encoded data 2751 of differential motioninformation and encoded data 2752 of a predicted motion informationposition. The differential motion information encoder 2702 receives theencoded data 2751 of differential motion information, and decodesdifferential motion information 2753. The adder 2706 adds thedifferential motion information 2753 to predicted motion information2756 (to be described later), and outputs motion information 2759. Thepredicted motion information position decoder 2703 receives the encodeddata 2752 of a predicted motion information position, and decodes apredicted motion information position 2754.

The predicted motion information selection switch 2705 receives thepredicted motion information position 2754, and selects the predictedmotion information 2756 from predicted motion information candidates2755. The predicted motion information position information 2560 isdecoded by using fixed-length decoding or variable-length decodinggenerated from the number of predicted motion information candidates2755. Variable-length decoding may also be performed using thecorrelation with adjacent blocks. Furthermore, if a plurality ofpredicted motion information candidates 2755 are identical, thepredicted motion information position information 2560 can be decodedbased on a code table generated from the total number of predictedmotion information candidates 2755 from which the identical candidatesare deleted. In addition, if the total number of predicted motioninformation candidates 2755 is 1, the corresponding predicted motioninformation candidate 2755 is determined as the predicted motioninformation 2556, so the predicted motion information positioninformation 2754 need not be decoded.

The arrangement and processing contents of the predicted motioninformation acquisition unit 2704 are the same as those of the predictedmotion information acquisition unit 901 explained in the firstembodiment.

The reference motion information acquisition unit 2704 receives thereference motion information 2558 and reference position information2560, and generates at least one predicted motion information candidate2755 2755A, 2755B, . . . ). FIGS. 10 and 11 illustrate examples of theposition of the predicted motion information candidate 2755 with respectto a decoding target prediction unit. FIG. 10 shows the positions ofprediction units spatially adjacent to the decoding target predictionunit. AX (X=0 to nA−1) indicates an adjacent prediction unit on the leftside of the target prediction unit, BY (Y=0 to nB−1) indicates anadjacent prediction unit above the target prediction unit, and C, D, andE respectively indicate adjacent prediction units at the upper rightcorner, upper left corner, and lower left corner of the targetprediction unit. FIG. 11 shows the position of a prediction unit in analready decoded reference frame with respect to the decoding targetprediction unit. In FIG. 11, Col indicates a prediction unit, whichexists in the same position as that of the decoding target predictionunit, in the reference frame. FIG. 12 shows an example of a listindicating the relationship between the block positions and indicesMvpidx of a plurality of predicted motion information candidates 2755.Indices 0 to 2 of Mvpidx indicate the predicted motion vector candidates2755 positioned in the spatial domain, and index 3 of Mvpidx indicatesthe predicted motion vector candidate 2755 positioned in the temporaldomain. Prediction unit position A is the position of a prediction unitof inter-prediction, i.e., a prediction unit having the reference motioninformation 2558, in AX, as shown in FIG. 10, and the value of X issmallest at prediction unit position A. Prediction unit position B isthe position of a prediction unit of inter-prediction, i.e., aprediction unit having the reference motion information 2558, in BY, asshown in FIG. 10, and the value of Y is smallest in prediction unitposition B. When prediction unit position C is not inter-prediction, thereference motion information 2558 in prediction unit position D isreplaced with the reference motion information 2558 in prediction unitposition C. When prediction unit positions C and D are notinter-prediction, the reference motion information 2558 in predictionunit position E is replaced with reference motion information 2558 inprediction unit position C.

If the size of the encoding target prediction unit is larger than thatof the smallest prediction unit, prediction unit position Col may storea plurality of items of reference motion information 2558 in thetemporal-domain reference motion information memory 2507. In this case,the reference motion information 2558 in the prediction unit at positionCol is acquired in accordance with the reference position information2560. The acquisition position of the reference motion information 2558in the prediction unit at position Col will be referred to as areference motion information acquisition position hereinafter. FIGS.13A, 13B, 13C, 13D, 13E, and 13F illustrate examples of the referencemotion information acquisition position for each encoding targetprediction unit size (32×32 to 16×16) when the reference positioninformation 2560 indicates the center of the prediction unit in positionCol. In FIGS. 13A, 13B, 13C, 13D, 13E, and 13F, each block indicates a4×4 prediction unit, and a circle indicates the position of a 4×4prediction unit to be acquired as the predicted motion informationcandidate 2755. FIGS. 14A, 14B, 14C, 14D, 14E, and 14F illustrate otherexamples of the reference motion information acquisition position.Referring to FIGS. 14A, 14B, 14C, 14D, 14E, and 14F, no 4×4 predictionunit exists in the position of a circle, so the predicted motioninformation candidate 2755 is generated by a predetermined method suchas the average or median of the reference motion information 2558 infour 4×4 prediction units adjacent to the circle. As still anotherexample of the reference motion information acquisition position, thereference motion information 2558 of a 4×4 prediction unit positioned atthe upper left corner of the prediction unit in position Col may also beused as the predicted motion information candidate 2755. In a case otherthan the abovementioned examples, it is possible to generate thepredicted motion information candidate 2755 by using any position andany method, provided that the method is a predetermined method.

Note that if no reference motion information 2558 exists, the motioninformation 2559 having a zero vector is output as the predicted motioninformation candidate 2755.

By the processing described above, at least one predicted motioninformation candidate 2755 is output from the reference motion block. Ifthe reference frame number of the aforementioned predicted motioninformation candidate 2755 is different from that of the encoding targetprediction unit, the predicted motion information candidate 2755 can bescaled by using the reference frame number of the predicted motioninformation candidate 2755 and that of the encoding target predictionunit. The predicted motion information selection switch 2705 selects oneof the plurality of predicted motion information candidates 2755 inaccordance with the predicted motion information position 2754, andoutputs the predicted motion information 952.

The inverse quantizer 2502 obtains a restored transform coefficient 2552by inversely quantizing the quantized transform coefficient 2551 fromthe entropy decoder 2501. More specifically, the inverse quantizer 2502performs inverse quantization in accordance with the information ofquantization decoded by the entropy decoder 2501. The inverse quantizer2502 outputs the restored transform coefficient 2552 to the inverseorthogonal transformer 2503.

The inverse orthogonal transformer 2503 performs, on the restoredtransform coefficient 2552 from the inverse quantizer 2502, inverseorthogonal transformation corresponding to orthogonal transformationperformed on the encoding side, thereby obtaining a restored predictionerror signal 2553. The inverse orthogonal transformer 2503 inputs therestored prediction error signal 2553 to the adder 2504.

The adder 2504 generates the decoded image signal 2554 by adding therestored prediction error signal 2553 to the corresponding predictedimage signal 2556. The decoded image signal 2554 is processed by adeblocking filter (not shown) or Wiener filter (not shown), temporarilystored in the output buffer 2511 for an output image, and also saved inthe reference image memory 2505 for a reference image signal 2555. Thedecoded image signal 2554 saved in the reference image memory 2505 isreferred to, as the reference image signal 2555, for each frame or eachfield as needed by the inter-predictor 2506. The decoded image signal2554 temporarily stored in the output buffer 2511 is output inaccordance with an output timing managed by the decoding controller2510.

The inter-predictor 2506 performs inter-prediction by using thereference image signal 2555 saved in the reference image memory 2505.More specifically, the inter-predictor 2506 acquires the motioninformation 2559 containing a motion difference (motion vector) betweenthe prediction target block and reference image signal 2555 from theentropy decoder 2501, and generates an inter-predicted image byperforming interpolation (motion compensation) based on this motionvector. The generation of the inter-predicted image is the same as thatin the first embodiment, so a repeated explanation will be omitted.

The decoding controller 2510 controls each element of the image decodingapparatus shown in FIG. 25. More specifically, the decoding controller2510 outputs the reference position information 2560 (to be describedlater) to the entropy decoder 2501, and performs various kinds ofcontrol for the decoding process including the above-describedoperations.

<Explanation of Skip Mode, Merge Mode, and Inter Mode>

The image decoding apparatus 2500 according to this embodiment uses aplurality of prediction modes using different decoding processes, asshown in FIG. 8. Referring to FIG. 8, the skip mode is a mode thatdecodes only a syntax pertaining to the predicted motion informationposition 2754 (to be described later), and does not decode any othersyntaxes. The merge mode is a mode that decodes only a syntax pertainingto the predicted motion information position 2754 and the transformcoefficient information 2551, and does not decode any other syntaxes.The inter mode is a mode that decodes a syntax pertaining to thepredicted motion information position 2754, the differential motioninformation 2753 (to be described later), and the transform coefficientinformation 2551. These modes are switched by the prediction information2571 controlled by the decoding controller 2510.

Also, the image decoding apparatus shown in FIG. 25 uses syntaxes thatare the same as or similar to the syntaxes explained with reference toFIG. 22, so a detailed explanation thereof will be omitted.

<Details of Motion Information Compressor 2508>

Next, the motion information compressor 2508 will be explained withreference to the flowchart shown in FIG. 16. When a process of decodinga frame (or an arbitrary unit such as a slice or coding unit) iscomplete, the motion information compressor 2508 compresses the motioninformation 2559 and stores the motion information 2559 in thetemporal-domain reference motion information memory 502.

First, the reference position information 2560 is acquired from thedecoding controller 2510 (step S1601), and a frame is divided intomotion information compression blocks as compression units of the motioninformation 2559 (step S1602). The motion information compression blockis a pixel block larger than a unit (typically, a block of 4×4 pixels)by which the motion information 2559 is stored by the motioncompensation process, and is typically a block of 16×16 pixels. Themotion information compression block may also be a block of 32×32pixels, a block of 8×8 pixels, a rectangular pixel block, or a pixelregion having an arbitrary shape.

Then, a representative motion information position is generated inaccordance with the reference position information 2560 (step S1603). Asan example of generating a representative motion information position,when the motion information compression block is a block of 16×16pixels, the reference motion information acquisition position when theprediction unit size is 16×16, as shown in FIGS. 13D, 14D, and 17D, isset as the representative motion information position. Subsequently, thereference motion information 2558 of the generated representative motioninformation position is set as representative motion information (stepS1604), and the representative motion information is stored in thetemporal-domain reference motion information memory (step S1605). StepsS1604 and S1605 described above are executed for all motion informationcompression blocks.

When the unit for storing the motion information 2559 is an M×M blockand the size of the motion information compression block is N×N (N is amultiple of M), the capacity of the reference motion information memorycan be reduced to (M×M)/(N×N) by executing the aforementioned motioninformation compression process.

<Other Embodiments of Representative Motion Information Position>

As another example of generating a representative motion informationposition, the central position of a plurality of reference motioninformation acquisition positions may also be used as the representativemotion information position. FIGS. 18A and 18B show the representativemotion information position of each motion compression block having asize of 16×16. FIG. 18A shows representative motion informationpositions when reference motion information acquisition positions arethe positions shown in FIG. 13D. Likewise, FIG. 18B shows representativemotion information positions when reference motion informationacquisition positions are the positions shown in FIG. 17D. Symbols ● inFIGS. 18A and 18B indicate reference motion information acquisitionpositions when a prediction unit is a 16×16 block. A representativemotion information position indicated by × is placed in the centralposition (also called the barycentric position) of four reference motioninformation acquisition positions.

As still another example of generating a representative motioninformation position, it is also possible to use a reference motioninformation acquisition position for each size of a plurality ofprediction units as the reference position information 2560, andgenerate a representative motion information position from a pluralityof reference motion information acquisition positions. FIG. 19 shows thecenter (reference motion information acquisition position) of eachprediction unit having a prediction unit size of 16×16 or more when atree block is a block of 64×64 pixels.

As another example of generating a representative motion informationposition, a representative motion information position can also be setby using a reference motion information acquisition position set foreach motion information compression block. FIG. 20A shows an examplewhen the barycenter of a plurality of reference motion informationacquisition positions in each motion information compression block isset as a representative motion information position. If the barycentricposition does not match the position of a 4×4 block, it is possible toset the nearest 4×4 block as a representative motion informationposition, or generate the reference motion vector 166 of the barycentricposition by using interpolation such as bi-linear interpolation.

FIG. 20B shows an example in which one of a plurality of referencemotion information acquisition positions is selected for each motioninformation compression block, and set as a representative motioninformation position.

Furthermore, FIGS. 21A and 21B illustrate examples when motioninformation compression blocks have the same reference motioninformation acquisition position in a tree block. Since all motioninformation compression blocks have the same representative motioninformation position, representative motion information positions neednot be switched in accordance with a position in the tree block. Therepresentative motion information position need not be any of thoseshown in FIGS. 21A and 21B. The representative motion informationposition can be any position such as the upper left corner or upperright corner in the motion information compression block.

As an example of generating a representative motion informationposition, a representative motion information position may also beindicated by using BlkIdx indicating positions of 4×4 blocks in a motioninformation compression block in Z-scan order. When the size of themotion information compression block is 16×16, the representative motioninformation position, as shown in FIG. 21A, is equivalent to theposition of BlkIdx=12. Also, the representative motion informationposition, as shown in FIG. 21B, is equivalent to the position ofBlkIdx=15.

As another example of the motion information compression process, themotion information compression process may also include reference framenumbers in order to reduce the memory capacity for reference framenumbers. In this case, a reference frame number stored in arepresentative motion information position is stored in the memorycapacity for reference frame numbers. Accordingly, the spatial-domainreference motion information memory 501 and spatial-domain referencemotion information memory 502 shown in FIG. 5 store reference framenumbers in addition to the motion vector information.

As still another example of the motion information compression process,when the motion information compression process does not include anyreference frame number, motion vector information in motion informationin a representative motion information position can be scaled by using areference frame number, and stored in the motion information memory 110.A typical example of the scaling process is a linear scaling processbased on reference frame number zero. That is, if the reference framenumber is not zero, the linear scaling process is performed such thatthe motion vector information refers to a reference frame correspondingto reference frame number zero. The basis of the above-described scalingprocess may also be a value other than reference frame number zero. Ifdivision occurs when performing the above-described linear scalingprocess, this division can be implemented by looking up a divisionprocess table formed beforehand.

When the motion information compression block is not a 16×16 block, arepresentative motion information position is generated by using thesame processing as described above. As an example, when the size of themotion information compression block is 64×64, a reference motioninformation acquisition position in a 64×64 prediction unit is set as arepresentative motion information position. As another example, arepresentative motion information position in the 16×16 motioninformation compression block shown in, for example, FIGS. 21A and 21Bcan be scaled in the horizontal and vertical directions in accordancewith the motion information compression block size, and the scaledposition can be set as a representative motion information position.

If no reference motion information exists because a representativemotion information position is outside a picture or slice, a positionwhere reference motion information can be acquired in a motioninformation compression block, for example, the upper left corner of themotion information compression block can be set as a new representativemotion information position. Also, if no reference motion informationexists because a representative motion information position is a regionto which intra-prediction is applied, a new representative motioninformation position can be set by executing the same processing.

Modifications of each embodiment will be explained below.

In each of the first and second embodiments, an example in which a frameis divided into rectangular blocks having a size of, for example, 16×16pixels and encoding or decoding is performed in order from the upperleft block to the lower right block on a screen has been explained (seeFIG. 2A). However, the encoding order and decoding order are not limitedto this example. For example, encoding and decoding may also beperformed in order from the lower right block to the upper left block,or spirally performed from the center to the edges of a screen.Furthermore, encoding and decoding may also be performed in order fromthe upper right block to the lower left block, or spirally performedfrom the edges to the center of a screen.

The first and second embodiments have been explained by showingprediction target block sizes such as a block of 4×4 pixels, a block of8×8 pixels, and a block of 16×16 pixels as examples, but a predictiontarget block need not have a uniform block shape. For example, theprediction target block (prediction unit) size can also be a block of16×8 pixels, a block of 8×16 pixels, a block of 8×4 pixels, or a blockof 4×8 pixels. Also, all block sizes need not be the same in one codingtree unit, and a plurality of different block sizes can coexist. When aplurality of different block sizes coexist in one coding tree unit, acode amount for encoding or decoding division information increases asthe number of divided blocks increases. Therefore, a block size isdesirably selected by taking account of the balance between the codeamount of division information and the quality of a locally decodedimage or decoded image.

In the first and second embodiments, a comprehensive explanation hasbeen described for color signal components without distinguishingbetween a luminance signal and color difference signals for the sake ofsimplicity. However, when different prediction processes are used for aluminance signal and color difference signals, it is possible to use thesame prediction method or different prediction methods. When differentprediction methods are used for a luminance signal and color differencesignals, a prediction method selected for the color difference signalscan be encoded or decoded by the same method as that for the luminancesignal.

In the first and second embodiments, a comprehensive explanation hasbeen described for color signal components without distinguishingbetween a luminance signal and color difference signals for the sake ofsimplicity. However, when different orthogonal transformation processesare used for a luminance signal and color difference signals, it ispossible to use the same orthogonal transformation method or differentorthogonal transformation methods. When different orthogonaltransformation methods are used for a luminance signal and colordifference signals, an orthogonal transformation method selected for thecolor difference signals can be encoded or decoded by the same method asthat for the luminance signal.

In the first and second embodiments, a syntax element not defined in theembodiments can be inserted between lines of the table indicating thesyntax configuration, or another description pertaining to conditionalbranch may also be included. It is also possible to divide the syntaxtable into a plurality of tables, or integrate a plurality of tablesinto one table. Furthermore, it is not always necessary to use the samelanguage, and freely change the language in accordance with a use form.

As has been explained above, each embodiment can implement efficientorthogonal transformation and inverse orthogonal transformation whilereducing the complexity in a hardware package and software package.Accordingly, each embodiment increases the encoding efficiency andimproves the subjective image quality.

Also, the instructions indicated by the procedures explained in theabove-described embodiments can be executed based on a program assoftware. A general-purpose computer system can obtain effects similarto those of the image encoding apparatus and image decoding apparatus ofthe above-described embodiments by prestoring the program and readingit. The instructions described in the abovementioned embodiments arerecorded, as a program executable by a computer, on a magnetic disk (forexample, a flexible disk or hard disk), an optical disk (for example, aCD-ROM, CD-R, CD-RW, DVD-ROM, DVD±R, or DVD±RW), a semiconductor memory,or a recording medium of this kind. The storage format of the recordingmedium can take any form, provided that the medium can be read by acomputer or an embedded system. The computer can implement the sameoperations as those of the image encoding apparatus and image decodingapparatus of the above-described embodiments by reading the program fromthe recording medium, and causing a CPU to execute, based on theprogram, the instructions described in the program. The computer can, ofcourse, also acquire or read the program across a network.

In addition, based on the instructions of the program installed in thecomputer or embedded system from the recording medium, an operatingsystem (OS) running on the computer, database management software, ormiddleware (MW) of the network or the like can execute a part of eachprocessing for implementing the embodiments.

Furthermore, the recording medium according to the embodiments is notlimited to a medium independent of the computer or embedded system, andincludes a recording medium storing or temporarily storing the programtransmitted across a LAN or the Internet and downloaded. It is alsopossible to store the program for implementing the process of eachembodiment in a computer (server) connected to a network such as theInternet, and download the program to a computer (client) across thenetwork.

Also, the recording medium is not restricted to one. Even when theprocess of each embodiment is executed from a plurality of media, thesemedia are included in the recording medium according to the embodiments,and the media can take any form.

Note that the computer or embedded system according to the embodimentsis used to execute each process of the embodiments based on the programstored in the recording medium, and can be, for example, a singleapparatus such as a personal computer or microcomputer, or a systemobtained by connecting a plurality of apparatuses across a network.

Note also that the computer according to the embodiments is not limitedto a personal computer, and includes a processing unit or microcomputerincorporated into an information processing apparatus. That is, thecomputer is a general term of an apparatus and device capable ofimplementing the functions of the embodiments based on the program.

Functional configurations of the image encoding apparatus and the imagedecoding apparatus according to each of the embodiments have beendescribed with reference to the drawings. The functional configurationsare implemented by hardware configurations, for example, as will bedescribed below.

An example of the hardware configurations of the image encodingapparatus and the image decoding apparatus according to each of theembodiments will be described with reference to FIG. 28. The imageencoding apparatus and the image decoding apparatus each comprise acontrol unit 2801, such as a CPU (Central Processing Unit) whichcontrols the overall apparatus, a main storage 2802, such as a ROM (ReadOnly Memory) or a RAM (Random Access Memory) which stores various dataor programs, an auxiliary storage 2803, such as an HDD (Hard Disk Drive)or a CD (Compact Disk) drive which stores various data or programs, anda bus connecting these elements. This is a hardware configurationutilizing a conventional computer. Further, the image encoding apparatusand the image decoding apparatus are connected wirelessly or through awire to a communication I/F (Interface) 2804 which controlscommunication with an external apparatus, a display 2805 which displaysinformation, and an operating unit 2806, such as a keyboard or a mousewhich receives instructions input by the user. Data to be encoded anddata to be decoded may be stored in the HDD, or input by the disk driveapparatus, or input externally via the communication I/F 2804.

The hardware configuration shown in FIG. 28 is a mere example. The imageencoding apparatus and the image decoding apparatus of each embodimentmay be implemented partly or entirely by an integrated circuit such asan LSI (Large Scale Integration) circuit or an IC (Integrated Circuit)chip set. The functional blocks of the image encoding apparatus and theimage decoding apparatus may be individually formed of a processor, ormay be integrated partly or entirely as a processor. Integration of thecircuits of the configuration is not limited to LSI, but may beimplemented as a dedicated circuit or a general-purpose processor.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. An image decoding method of dividing an inputimage signal into first pixel blocks, and performing inter-prediction onthe divided first pixel blocks, comprising: selecting predicted motioninformation from a plurality of items of motion information in a decodedregion; predicting the selected predicted motion information of adecoding target block; and acquiring representative motion informationfrom a second pixel block which exists in a center of region in a sameposition of the decoding target block in a reference frame, inaccordance with first information specifying the center of the region inthe same position of the decoding target block, the first informationbeing selected from a plurality of items of motion information in thedecoded region, wherein the representative motion information includesmotion information at a position of representative motion information ofthe second pixel block, the first information includes secondinformation which specifies a position in the decoding target block, thesecond information being used to select the motion information, and therepresentative motion information includes the motion information, whichis positioned in a center of a plurality of items of the secondinformation of the decoding target block and an adjacent pixel block. 2.The method according to claim 1, further comprising: performing ascaling process in accordance with information of a reference framecontained in the representative motion information, and storing a resultin the motion information buffer; and performing a scaling included inthe representative motion information, in accordance with information ofthe reference frame contained in the predicted motion information. 3.The method according to claim 1, wherein the representative motioninformation contains information of a reference frame.
 4. The methodaccording to claim 1, further comprising, if the decoding target blockis selected from a plurality of block sizes, generating thirdinformation which specifies a position in the decoding target block, thedecoding target block corresponding to the block sizes, from a pluralityof items of the second information corresponding to the block sizes,wherein the first information contains the third information.
 5. Themethod according to claim 4, wherein one of (A) position informationspecified by one of the items of the second information, and (B)position information corresponding to a center of the items of thesecond information is set in the third information.
 6. An image decodingapparatus for dividing an input image signal into first pixel blocks,and performing inter-prediction on the divided first pixel blocks,comprising: a selection unit to select predicted motion information froma plurality of items of motion information in a decoded region; aprediction unit to predict the selected predicted motion information ofa decoding target block; and an acquisition unit to acquirerepresentative motion information from a second pixel block which existsin a center of region in a same position of the decoding target block ina reference frame, in accordance with first information specifying thecenter of the region in the same position of the decoding target block,the first information being selected from a plurality of items of motioninformation in the decoded region, wherein representative motioninformation includes motion information at a position of representativemotion information of the second pixel block, the first informationincludes second information which specifies a position in the decodingtarget block, the second information being used to select the motioninformation, and the representative motion information includes themotion information, which is positioned in a center of a plurality ofitems of the second information of the decoding target block and anadjacent pixel block.